Conventionally, auto-leveling control can adjust automatically an optical axis position of a vehicle headlamp according to a tilt angle of a vehicle, thereby changing an irradiating direction. Generally, in the auto-leveling control, a vehicle height sensor is used as a vehicle inclination detecting device. An optical axis position of a vehicle headlamp is adjusted, based on a vehicle pitch angle detected by the vehicle height sensor. In contrast with this, JP-A-2000-085459, JP-A-2004-314856, JP-A-2001-341578, and JP-A-2009-126268 disclose configurations for implementing the auto-leveling control using an acceleration sensor as the inclination detecting device.
In an implementation where an acceleration sensor is used as the vehicle inclination detecting sensor, an auto-leveling system can be provided at lower cost and lower weight, as compared with an implementation where a vehicle height sensor is used as the vehicle inclination detecting sensor. On the other hand, even in an implementation using the acceleration sensor, there is a request for performing the auto-leveling control with high accuracy by reducing errors in detection by the sensor.